A cytoplasmic organelle that is required for the proper formation of germ line syncytia during gametogenesis in both male and female insects. The fusome arises from endoplasmic reticulum that traverses the ring canals (q.v.) formed during successive cycles of incomplete mitotic germ cell divisions. After each division, a plug of fusomal material accumulates in each newly-formed ring canal. This material then fuses with the fusome(s) formed from the previous division(s), and ultimately a mature, branched structure called a polyfusome is produced. The polyfusome entry illustrates how this structure directs the pattern of cystocyte interconnections by anchoring one pole of each mitotic spindle, thus orienting the plane of cell division. The first Drosphila mutant shown to have fragmented fusomes was called otu because it formed ovarian tumors made up of hundreds of cells, most of which were not connected by ring canals and which never differentiated into either oocytes or nurse cells. Mitotic effectors such as cyclin A (q.v.) have been shown to bind transiently with fusomes during G2 and prophase, and this suggests that the fusomal system plays a role in the timing, synchronization, and eventual cessation of cystocyte divisions (q.v.). Among the other components identified in fusomes are alpha and beta spectrins, ankyrin, an adducin-like protein, dynein, and a protein encoded by the bag of marbles gene. Mutations in this gene also produce ovarian tumors. See adducin, bag of marbles (bam), hu-li tai shao (hts).
Subjects: Genetics and Genomics.